1. Field of the Invention
This invention relates to improvements in or relating to the operation of systems for raising net-type aircraft arresting gears.
At present, on take-off and landing strips or runways these arresting gears consist mainly of a net adapted to be raised in front of the aircraft in case of emergency, or alternatively of a cable extending across the strip, in the case of aircraft equipped with a special crosshead.
It is the essential object of the present invention to widen the field of possible uses for the net raising system proper.
2. Description of the Prior Art
The most popular systems employed for this purpose consist in raising a net disposed flat on the ground across the strip by means of posts driven in turn by a rope secured to a pneumatic cylinder controlled either from the control tower or from the barrier itself. These cylinders are of the high-pressure type and operate as a rule under pressures of the order of 40 bars, and each post is swivel-mounted on a kind of universal joint to permit its movement in all directions. In the raised position the posts are braced by additional ropes anchored to small concrete blocks. Now this arrangement requires the use of several concrete blocks, as a rule six on each side of the runway, and also precise adjustments of the various components, frequent inspections and considerable maintenance, since certain components are symmetrical. On the other hand, the energy stored is relatively high and the cylinder power requirements are of the order of 12 tons, and a 2-ton stress is exerted permanently on various other components of the structure.
Other inconveniences are also observed. Thus, the high-pressure cylinder operation reduces considerably the number of times the posts can be raised (at the most five times with two air cylinders delivering seven thousand liters of expanded air), so that the compressed air cylinders must be replaced rather frequently, and the barrier cannot be used during such replacements. It is also impossible, on the other hand, to control the lowering of the barrier from the control tower, and also to refill the reservoir in the same manner. However, these operations must be accomplished on the site and require a crew of at least six men.
Obviously the cost of these known systems is relatively high, considering the number of component elements involved. High-pressure operation further involves the use of measuring and control instruments such as pressure-reducing valves, high-pressure control switches, etc. which, though mass-produced, are still very expensive devices.